World Resource Review
Vol. 16 No.2
CAPTURING CARBON DIOXIDE DIRECTLY
ATMOSPHERE
FROM THE
FrankS. ZemanandKlaus S. Lackner
Departmentof Earth and EnvironmentalEngineering
ColumbiaUniversity
918 Mudd, MC 4711
500 West 120dtStreet
New York, NY, 10027 USA
Email: [email protected]
Keywords: Air extraction,COb sequestration,climate change,kraft process
SUMMARY
The increasingconcernoverrising CO2levels in the atmosphereand
their potentialclimateeffectsis fuelling researchaimedat carbonmanagement.
One areaof researchfocuseson capturingthe CO2after combustionand
sequesteringit underground.Captureschemeswould operateat the site of
generationtaking advantageof the elevatedconcentrationsof CO2in the effluent.
Here,however,we proposean indirectmethodof carboncapturethatremoves
CO2from the atmosphere.This processcombinesexistingtechnologieswith
recenttechnologicalinnovationsinto a novel carboncaptureconcepttermedair
extraction. The processusesdissolvedsodiumhydroxideto removethe CO2from
ambientair. The resultantsodiumcarbonatesolutionis causticizedusing calcium
hydroxideto regeneratethe sodiumhydroxide solutionandprecipitatecalcite.
The calciteis then thermallydecomposed
to producelime andCO2.The lime is
hydratedto completethe process.Theselatterstagesareusedroutinely in the
paperindustryandthe calcinationof limestoneis centralto the cementindustry.
This paperreviewsthe processand highlightspertinentresearchto developa
likely cost-effectiveprocess. It is shownthatthe proposedprocessis well defmed
and technicallyfeasible.The wide parameterspaceand potentialfor
improvementssuggestthat further efficiency improvementsareattainable. The
most significantimprovementswill be derived from efficient heatmanagement.
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1 INTRODUCTION
Recentyearshave seenattentiondrawnto issuessun-oundingchangesin
the global climateinducedby humanactivities. Concernoverthesepotential
changeshasled to the formationof internationalagencies,suchasthe
IntergovernmentalPanelon ClimateChange(IPCC),whosemissionis to consider
this problem. The IPCC hasstatedthat carbondioxide (COJ is the greenhouse
gascontributingthe largestportion of the anthropogenicincreasein global
radiativeforcing, which the IPCC defmesasan externallyimposedperturbationin
the radiative energybudgetof the Earth's climatesystem. AnthropogenicCO2
producesapproximately62% of the increaseor 1.46 W1m2out of a total increase
of 2.43 W/m2(HoughtonandDing, 2001). The increasein atmosphericCO2
levelscanbe largelyattributedto the combustionof fossil fuels,with a small
contributionfrom cementproductionand landusechanges.The total global
emissionsreached6,611 million metric tons of carbonin 2000,which representsa
1.8%increaseover 1999(Marland,BodenandAndres,2003). Giventhe world's
economicdependenceon fossilfuels andthe expectedincreasein consumption,
methodsfor mitigating the atmosphericreleaseof CO2needto be developed.
The primary targetsfor mitigationare powerplantsas theyproducelarge
concentratedstreamsof CO2(Herzogand Drake,1996). Thesesourcesaccount
for aboutonethird of the worldwide CO2emissions.Even aftereliminating all
emissionsfrom power plants,the remainingtwo thirds would still be releasedto
the atmosphere.Roughlyhalf of all emissionsarisefrom smalldistributedand
oftenmobile sources. Mitigating their CO2impacton the atmosphereis more
difficult. One approachis to provide fuels that are carbonfree, which is the
reasonmuchemphasisis beingplacedon hydrogenasa transportationfuel.
Here,we proposean alternativemethodfor mitigating CO2emissionsfrom
sourcesotherthan powerplants. We proposethatCO2is removeddirectly from
the atmospherein a costeffective, industrialprocesshereafterreferredto as air
extraction.
Air extractioncanbe viewedasa variationof flue gasscrubbingwhere
the fluid is at atmospherictemperatureandpressurewith a CO2concentrationof
0.037%. The implementationof the processdiscussedhereusesa sodium
hydroxide (NaOH)based,alkalineliquid sorbentto removethe CO2from the
ambientair by producingdissolvedcarbonateions. In orderto recoverthe
sodiumhydroxide,the resultantsodiumcarbonate(NazCO3)solutionis mixed
with calciumhydroxide (Ca(OH)Jto producesodiumhydroxideand calcium
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with calcium hydroxide (Ca(OH)2)to producesodiumhydroxideand calcium
carbonate(CaCO3)in a reactionknown as causticizing. This reactiontransfers
the carbonateanion from the sodiumto the calciumcation. The calcium
carbonateprecipitates,leavingbehind a regeneratedsodiumhydroxidesorbent.
The precipitateis dried, washedand thermally decomposed
to producelime
(CaO). This step. knownas calcination, is followed by hydratingthe lime.
known as slaking, which completesthe process.The processof recycling of
sodiumhydroxide using calciumhydroxide hasbeenin usein the pulp and paper
industrysince 1884,where it is known asthe Kraft Process(Miner and Upton,
2002). This processalso involvesthe calcinationof limestone,which is at the
heart of the cementmanufacturingindustry. In short. eachunit processrequired
is known to be technicallyfeasibleand the only remaining questionssurround
their efficienciesand costs.
This paperwill presenta re'tiew of the indi~idual components
comprisingthe air extractionprocesswith a view to highlighting the potential
benefitsand concerns. The issuessurroundingtheir integrationinto a functional
air captureS}.stemwill also bediscussed.The layout of the paperwill follow the
path of the C~ moleculethroughthe system. A simplified schematicof the
processis shownin Figure 1.
Figure 1 Overviewof Air ExtractionProcess.Note that two reactionsare not shown;
!hying (d), and hydrating(h).
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2 ALKALINE SODIUM SORBENTS
The removalof a gaseouscomponentthrough contactwith a liquid is
known as wet scrubbihg. Wet scrubbingcanbe divided into processeswhere
there is a chemicalreactionbetweenthe sorbateandthe sorbentand wherethe
sorbateis physicallydissolvedinto the sorbentsolution. For the air extraction
processwe proposean alkaline sodiumsolventwhich reactschemicallywith the
entrainedCO2. The chemicalreactionfor this processis shownbelow as reaction
(1).
2NaOH (aq)+ CO2(g) -+ Na2CO3(aq)+ H2O;
(1:
The aqueouscarbonatereactioncanbe simplified by omitting the cation,
resulting in the following ionic reaction.
20H-(aq) + CO2 (g)-+ CO32-(aq)
+ H2O 0)
(2)
6.GO= -56.1 kJ/mol (Lllio= -109.4 kJ/mol)
Note thatthe enthalpyandfree energyof the reactionare for a nominal I
molar solution. The thermodynamicdata,given at298K anda pressureof 1 bar,
was obtainedfrom the availableliterature(Lide, 2000). As a comparison,the free
energyof mixing CO2with nitrogenand oxygento form ambientair is given by
~G = RT In (P atrnlPCoJ
-20 kJ/mol.
(3)
Clearly, sodiumhydroxideprovidesa sufficientdriving force to
effectively collectCO2from ambientair. Eventhougha lowerbinding energy
might be desirable,the high binding energyof chemicalsorbentsprovesuseful in
absorbingCO2from streamswith low partialpressuresof CO2(White, et al.,). As
an alternativewith a weakerbinding energy,one may considersodiumor
potassiumcarbonatebuffer solutionsasa sorbent.In this casethe absorptioncan
be describedby:
CO2(g)+ CO]2- + H2OQ) -+ 2 HCO]'
IlGO = -14.3 kJ/mol
(4)
(ilHO=-27.6 klimat)
Even in this caseit is possibleto mustera sufficientthermodynamic
driving force to removeCO2from the air. For a two molar solutionof bicarbonate
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ions, the free energyof the reactionfrom ambientair is negativeif the bicarbonate
concentrationstaysbelow0.15molar. A similar resultcanbe obtainedby
calculatingthe massactionequilibriumusing empiricalvaluesfor the equilibrium
constants.Reaction(4) is effectivelytrimolecularand is the result of a sequence
of reactionswhich have fastkinetics at high temperaturesor very high carbonate
to bi-carbonateratios. Otherwisethe processoccursin the diffusion regime,
which is much slower(Astarita, 1967),makingthis reactionimpracticalfor air
extractionasit is kinetically limited.
In a recentreviewfocusedon flue gas,White et al. (2003)do not
specificallydiscussabsorptionon sodiumhydroxide (NaOH),ratherthey consider
aqueoussolutionsof sodiumand potassiumcarbonate.Thesesorbentswere
comparedto mono-ethanolamine
(MEA), the industrystandard,and it was
concludedthat MEA providesa substantiallymore cost-effectivesolution(Leci
and Goldthorpe,1997). At thesehigherCO2concentrationstherewould be no
advantagein the higherbinding energyof the hydroxideandthe heatof this
reactionis in anycaseirretrievablylost. An additionalcomplicationarises
becausethe productcarbonatecannotbe decomposedwith eitherheator pressure.
The resultantNazCa] mustbe efficiently decomposedchemically. We proposeto
accomplishthe chemicaldecompositionusing calciumhydroxideasan
intermediary.
A sodiumhydroxide solutionprovidesa liquid sorbentthat is more easily
cycledthrougha piping systemthan a calciumhydroxidesuspension.Its binding
energyis strongenoughand its reactionkinetics fast enoughto obviatethe need
for heating,cooling or pressurizingthe air. BecauseCO2is so dilute any such
actionwould result in an excessiveenergypenalty. The hydroxide solutionavoids
all suchcomplications. Sincesodiumhydroxideis cheaperthan potassium
hydroxide our startingpoint for the air extractiondesignwill be basedon sodium
hydroxide.
2.1 Experimental Investigations into SodiumHydroxide Sorbents
The absorptionof CO2by NaOH solutionwas studiedin 1943by Tepe
andDodge(1943). Experimentswereperformedusing a packedtower
arrangementwith the inlet gashavinga CO2concentrationof -2%. Tepeand
Dodgeinvestigatedthe effectsofNaOH concentration,NazCO)concentration,gas
flow rate, solventflow rate,and solventtemperatureon the CO2uptakerate. The
effectswere quantified in terms of an over-all absorptionratecoefficient.The
resultsshowedthatthe rate coefficientincreasedwith increasingNaOH
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concentrationto a maximumnormality of -1.8 and thendecreased.The rate
coefficientdecreasedwith increasingN~CO3 concentration.The gas flow rate
had no effect, while the rate coefficientincreasedwith increasingsolventflow
rates. The temperatureof the solventaffectedthe rate coefficient significantly
with the rate coefficientfollowing the temperatureof the solventto the 6thpower.
The absenceof sensitivityto gas flow rate and high sensitivityto temperature
suggeststhat for theseCO2concentrations,
the reactionis limited by transport
resistancein the liquid phase.More recentresultson MEA solutionsalso show
that at elevatedCO2concentrationsabsorptionis limited by resistancein the liquid
phase(Aroonwilas et al.,2001).
Generallyin wet scrubbing,onecanbreakthe transportresistanceinto
two distinct components,
the air sideand liquid sideresistance.The air side
resistanceis dominatedby the diffusion barrier in the laminarboundarylayer.
Typically, sucha boundarylayeralsoexists onthe liquid side. In the bulk fluid,
dissolvedCO2reactswith water,or hydroxideionsto form carbonateor
bicarbonateions. In contrastto the reactionsof CO2with water,the reactions
with hydroxidereactionsare very fastand their reactiontime canbe ignored
(Astarita,1967). However,sincediffusion coefficientsof CO2in air are roughly
four ordersof magnitudelargerthan ionic diffusion coefficientsin water, it is easy
to becomeratelimited on the liquid side. The experimentsof TepeandDodge
suggestthatthe transportresistanceis dominatedby the liquid phase.
For a onemolar carbonateion concentrationin the liquid, the
concentrationratio betweencarbonateions in the fluid andCO2moleculesin the
gasis 66,000: I. It thus will taketime to fill up a boundarylayer on the liquid
side. This suggeststhat at sufficiently low partialpressuresof CO2the extraction
processwill be limited by air~sideresistance.
The absorptibnof CO2from atmosphericair usingan apparatussimilar to
that of TepeandDodgewasstudiedby SpectorandDodge (SpectorandDodge,
1946). For ambientair, CO2absorptionrateswere proportionalto G", whereG is
the air flow rateandthe coefficienta.varies from 0.35 at low flow ratesto 0.15 at
high flow rates.
This suggeststhat at low CO2concentrations,
0.031%, the liquid side
resistanceto transportceasesto be dominant. It is likely that in theseexperiments,
fluid surfaceregenerationwas sufficiently fastto preventa built up of liquid-side
flow resistance.The experimentsdid showthat wet scrubbingusingNaOH can
effectivelyremoveCO2from atmosphericair at a ratewherethe unavoidableair
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sideresistanceto flow hasbecomeimportant. SpectorandDodgeobtained
approximately90%removal for atmosphericconcentrationlevels.
The advantageof usinga stronghydroxide for CO2captureis a high load
capacityanda fast reactiontime. The resultsby SpectorandDodge suggestthat a
systemcanbe built that will be limited by transportresistancein the air side of the
air-liquid contactsurface.
In a regimewherethe dominanttransportresistanceis on the air side,it is
possibleto estimatethe sizeof the CO2extractorby the air dragthe extractor
causeson the flow. Apart from the pressuregradientdriven momentumflow,
momentumtransferto the wettedsurfacefollows a similar transportequationas
the CO2diffusion. As a consequence,
a systemthat incursa pressuredrop roughly
equalto pV2,which extractedvirtually all of the initial momentum,will be ableto
extracta substantialfraction of the CO2from the flow. To setthe scaleof the
operation,at 10m/sthe air flow throughan openingof 1m2carriesa CO2load that
equalsthe CO2producedby generating70 kW of heatfrom coal(Lackner,Grimes
andZiock, 1999). A 100MW powerplant operatingat 33% efficiencywould
require9,000m2of wind crosssection,if CO2collectionefficiencywould be about
50%.
3 CAUSTICIZATION
Causticizationrefersto the transfonnationof sodiumcarbonateinto
sodiumhydroxide. It is generallyperfonnedby addingsolid calciumhydroxide
to the sodiumcarbonatesolution. The solubility of calciumhydroxideis suchthat
this fonDSan emulsionaccordingto reaction(5).
(5)
This reactioncanalsobe written in its ionic form asfollows.
(6)
dGO = -18.2 kJ/mol
(L\HO= -5.3 kJ/mol)
This process step regeneratesthe sodium sorbent. The CO2 is removed as
a solid through a filtration process. Zsako (1998) noted that one could also start
with lime, which would slake immediately in the aqueous solution, but in air
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extraction,it is importantrecoverthe heatof the slakingreactionat elevated
temperatures.Thuswe haveseparatedthe slaking stepfrom the causticization.
Experimentshave shownthatthe causiticizationrate increaseswith
temperature(DotsonandKrishnagopalan,1990). The initial sodiumcarbonate
concentrationfor thoseexperimentswas -2.0 mol/l andthe sampleswere
subjectedto constantstirring. Reaction(5) eventuallyapproachesequilibriumand
causticizingefficiency is generallyin the rangeof 80 to 90%. Causticizing
efficiency refersto the amountof sodiumcarbonateconvertedto sodium
hydroxide. Dotson(1990)determinedthatthe rate constantfor the reaction(5)
increasedby a factor3 asthe operatingtemperaturewasraisedfrom 353 to 393K.
The rate constantdroppedwhenthe feed solutioncontainedsodiumhydroxide.
The experimentalcausticizingefficiency for pure sodiumcarbonateand a mixture
of sodiumhydroxide and sodiumcarbonatewere -94% and -85% respectively.
The rate constantfor the causticizationis drivenby the concentrationof
free Ca ions in solution. Highly alkalinesolutionswill limit the availability of
dissolvedCa++at anytime and consequentlyreducethe rate of conversion.
Elevatedtemperaturesandactivestirring reducediffusionalresistanceandthus
will increasethe rate of reactions.
It was alsonotedthatthe concentrationsof all the calciumspeciesremain
essentiallyconstantthroughoutthe reactionsdueto their low solubility. This
suggeststhatthe efficiencyandrate constantsmaychangeif insufficientcalcium
is present. Dotsonpreventedthis occurrenceby usinga 10%stoichiometric
excessof lime. However,this excessresultsin solid calciumhydroxidebeing
entrainedwith the filtrate. This would producehigherenergyconsumptionin the
lime kiln dueto the dehydrationreactionmentionedby Zsako(1998).
The concentrationsof the variousspecies,both sodiumandcalcium,have
a profound effect on the quality of the resultantfiltrate. Konno et al. observed
thatthe solid phaseof calciteis unstablein pureNaOH solutiongreaterthan 2
moVl and easilyconvertsto Ca(OH)2(Konno, Yasunoriand Kitamura,2002).
Thesesolidsbecomestablein a I moVl NaOH solutioncontainingat least0.02
mol/l N~CO3. This latter solutionmixture suggeststhat for an initial sorbent
concentrationof I mol/L, 96% of the hydroxideions wereconvertedto carbonate
accordingto reaction(2). The presenceofN~CO3 alsoreducesthe solubility of
calcite. The solubility ofCa(OH)2is stronglydependentonthe NaOH
concentrationand dropsby a factor4, to 5 x 10-4moVl,asthe NaOH increases
from 0 to 0.5 moVl. Konno alsoobservedthe concentrationsofCa2+andNaOH
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during the reaction. As expectedthe Ca2+concentrationdroppedandthe NaOH
concentrationincreasedasthe reactionprogressed.The initial Ca2+concentration
was -1 x 10-3moM. Konno alsonotedthat Ca(OH)2supersaturationratio is the
driving force for nucleation. In effect,theseprocesses
balancethe solubility of
calciumhydroxide againstthe solubility of calciumcarbonate.The valuesfor the
dissociationconstantsareavailablein the literature(Snoeyinkand Jenkins,1980).
[Ca~[OH-r < KoH=10-'49mor/l
[Ca++][CO32-] < Kco3=10-3.22
mor/l
(8)
(9)
Given that the calciumconcentrationis the samein both (8) and (9), we
can solve for the carbonateconcentrationasfollows.
[CO]20]= <Kco]lKo~ x [ORoP
Assuminga 1 molar sodiumsolutionwe can neglectthe effectof calcium
on the chargebalanceandthe sodiumconcentrationmustthereforebalanceall the
negativeions. Ifwe furtherdefmethe causticizingefficiency (E)asthe ratio of
hydroxide ions over sodiumions we obtainthe following relationship.
-1
&=
(2~[OH-]+
KOH
The stablesolutionsuggestedby Konno would containapproximately1
mol per liter hydroxide ions. This would suggesta theoreticalcausticizing
efficiency of 96%, which is slightly higherthanthe experimentalvalue of Dotson
et al. (1990). The differenceis likely dueto the omissionof ionic activity in the
calculations.
The experimentalwork discussedaboveprovidesa pathwayfor
recoveringsodiumhydroxide from sodiumcarbonate.In the process,the carbon
dioxide has beentransferredinto a solid form of calciumcarbonate,which canbe
readilyremovedfrom the liquid and afterwashinganddrying it canbe thermally
decomposed.The causticizationtakesplace in an emulsionof calciumhydroxide.
Thereare a numberof options for the implementation,but this unit processis
well establishedin the pulp and paperindustry.
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4 CALCINATION OF LIMESTONE
The fmal stageof the air extractionprocessis the recycling of the calcite
precipitate. This is accomplishedthroughthermalregenerationor calcination.
Lime and limestoneareamongthe oldestmaterialsusedby humanitywith the
fIrst recordeduse in the Egyptianpyramids. The fIrst soundtechnicalexplanation
of lime calcinationcamein the 18thcenturyfrom the British chemist,JosephBlack
(Boynton, 1966). Therearethreeessentialfactorsin the kinetics of dissociation;
the dissociationtemperature,the durationof calcination,andthe CO2in the
surroundingatmosphere.The reactionis shownbelow.
CaCO3(s) -4 CaO(s) + CO2(g);
LiliO = + 179.2 kJ/mot
12)
The fIrSt quantificationof the thennaldecompositionwasperfonned in
1910andthe resultwas a decompositiontemperatureof 1171K in a 100%CO2
atmosphereat atmosphericpressure(Johnston,1910). Currentpracticesuselime
kilns to dissociatethe calcite which vary greatlyin their perfonnance.The most
importantperfonnancemetric for air extractionis the thennalefficiency, which is
the productof the theoreticalheatrequirementandthe availableoxide content
divided by the total heatrequirement.Boyntoncomparesthree hypotheticalkilns
and basesone on "the lowestfuel efficiency onrecord,that of the mostadvanced
Gennanmixed-feedkiln." This advancedkiln is reportedto obtaina thennal
efficiency of 85% for 93%availablelime. The thennalefficiencyrefersto the
proximity to the theoreticalminimum heatrequirementasdefmedby reaction(12)
while the availablelime refersto the amountof inert materialpresent,in this case
7%. This translatesinto a total heatrequirementof 3 .03 MMBtu per ton of lime
or 4.5 GJ per tonne of CO2,This latterfigure is lowerthanthe 4.8 GJ pertonne of
CO2used in a previousair extractionfeasibility study(Zeman,2003). The
thennodynamicminimum heatrequirementof 4.1 GJ/tonneCO2canbe calculated
from the enthalpyvalue in reaction(12).
The potentialcostof air extractionwill be dominatedby this reactionand
any improvementregardingthe threekinetic factorslisted abovewill directly
affectthe cost of the project. A lower dissociationtemperaturewill requireless
heatinput as will a shorterdurationof calcinationanda lower CO2contentin the
surroundings.Garcia.,Labianoet al. found that calcinationrate increasedwith
temperaturein a neutralenvironmentanddecreasedwith increasingCO2partial
pressureand total pressure(Garcia-Libianoet al., 2002). Khinasteta1.echoed
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similar fmdingsregardingthe effects of CO2partialpressurebut alsofound a
decreasein reactionrateswith increasingparticlesize (Khinastet al., 1996). The
effect of stearnon calcite decompositionwas alsoinvestigated.It was found that
stearnenhancedthe calcinationrate and evencompensatedfor the presenceof
CO2overthe solid (Thompsonand Wang,1995). Thomson'sexperiments
producedgreaterthan 95% conversionafter 40 minutesin an environmentof 21 %
stearnand 79% helium at a temperatureof 753 K. The positive effects of stearn
on calcinationmay be synergisticwith otheradvancements
in improving the
efficiency of calcination.In the caseof lime mud calcination,Thelianderproposed
a novel stearndrying systemthatreducesthe overall systementhalpyby 33%
(ThelianderandHanson,1993). Therearesomenovel designsfor calcination
unitsaswell. The decompositionof limestonewastestedin a solar reactorand
achievedan averageof -50% conversion(Imhof, 1997).
5 AIR EXTRACTION AS CARBON CAPTURE
We areproposingto captureCO2directly from the atmospherein a cost
effectivemanner. As such,a brief comparisonwith the industrystandardswill
provide a benchmarkfor future work. Stericallyhinderedamines(SHA) and
MEA areconsideredpotentiallysuccessfulCO2capturetechnologies.Theyare
regeneratedusing steamand their thermalenergyrequirementsare 700and 900
kcal/kg CO2for KS-2 andMEA, respectively(Mimura et al., 1997). Thesevalues
canbe convertedto 2.9 and3.8 GJ/tonneCO2. It shouldbe noted that Mimura et
al. captured90% of the CO2generatedby the powerplants;the remainderwould
haveto be mitigated by othermeans. An economicanalysisof CO2captureusing
MEA obtaineda cost of $50 pertonne of CO2avoided(Zappelli et al., 2003).
Calciumbasedsorbentshavealso beeninvestigatedfor use in CO2capture. The
thermalrequirementswill follow the limits outlinedabove;however,the
durability of the sorbentis alsoan importantcostfactor. This has beendiscussed
for dry CO2cycles.Abanadessummarizedseveralstudiesof the
carbonation/calcination
cycle and found very good agreement(Abanades,1998).
Starting at around80% conversion,performancerapidly droppedto lessthan 20%
conversionat 14cycleswhere it stabilized. Thesetestswere conductedunder
different conditionsthanproposedfor air extraction.Most importantlythe air
extractionprocesswould hydrateor slakethe resultinglime. Generallylime is
regeneratedin the hydrationprocess,thuswe do not expecta greatreductionin
captureefficiency from onecycle to the next. The processof hydrationis
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CaO(s)+ H2O (I) -+ Ca(OH)2(s)
LiliO = -64.5 kllmot
(13)
The hydrationcausesboth expansionandthe liberationof heatwhich
causesthe particleto split, exposingfreshsurfacesand therebyreducingthe
effectsof sintering. The inclusionof this reactionin the carbonationprocesswill
likely alterthe perfonnanceand durability of the lime cycle, as will the lower
temperaturesof reaction. We believethatthesedifferencesprecludethe useof
durability datapresentedby Abanadesasa deterrentfrom further study. Zsako
(1998)statedthat slakedlime undergoesdehydrationat temperaturesabove
~700K underambientconditions. This defmesthe rangeof possibleoperating
temperaturesfor the hydrationprocess.Hydrationis highly exothennicandcan
provide usefulheatenergyif it is perfonnedefficiently at high temperatures.
Thereare othermethodsfor removingCO2from the atmosphere
including physicalabsorptionand refrigerationamongothers. Steinberg
comparedeight differentmethodsof CO2removaland found alkaline wet
scrubbingto be the leastenergyintensive(Steinbergand Dang,1977). The
estimatedenergyrequiredto strip air of CO2was 0.4 kWh(e/lbmethanolor 4.4
GJ/tonneCO2. The CO2was strippedusing a 0.0I N K2CO3solutionandwas
desorbedby hydrogen,from electrolysis,and low pressuresteam. It shouldbe
noted thatthis calculationdid not includelossesduring the conversionof primary
fuel to electricity.
The feasibilityof air extractionwill dependon the overall cost in
comparisonto alternateremovaltechnologies.The costper unit removalwill
furtherdependonthe energyrequirements,the durability of the sorbents,and
costsexternalto the process.Theseexternalcostscould includeexcessivewater
lossesfrom the wet scrubbing.As this part of the processwill be in contactwith
the openatmosphere,evaporationcanbe expected.This canbe minimized by
adjustingthe sorbentconcentrationwhich variesthe vaporpressureuntil it
matchesthat of the ambientair. The thennophysicalpropertiesof sodium
hydroxide solutionare known for a wide rangeof concentrations(Olsson,
Jernqvistand Aly, 1997). Otherexternalcostswill be delineatedduring bench
and pilot scaledemonstrations.The calcinationreactionis likely the most energy
intensivefor the statedprocess.This highlightsthe needfor efficient heat
managementwithin the system. Additionally, any significantlime degradation~
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..
6 CONCLUSION
Our previous feasibility study mentioned earlier obtained a price range of
~$25- 75 per tonne of CO2extracted (Zeman, 2003). The large range is due to the
fluctuations in the price of natural gas and the uncertainty regarding the cost of
solid oxide membranes. This range does match up with the quoted cost of MEA
capture and does suggest further studies may provide a viable option for
extracting CO2 from the atmosphere. Air extraction is not necessarily more
expensive than MEA capture in a flue stack. Even though the air contactors must
be much larger than the equivalent contact surfaces in the flue stack, their
contribution to the total cost may be very small. The recovery MEA sorbents
requires similar amounts of energy, and becausethe air is clean and hydroxides
are not subject to oxidative losses, the make-up costs are low in a hydroxide
system. It is however clear, that the biggest challenge for air extraction will lie in
the efficient management of heat and not in the design of the physical reactions.
The information highlighted in this paper maps out a large parameter
space. We believe that such a broad discussion will prove useful in designing new
implementations of processes that have been optimized in the past for entirely
different goals. Our objective is quite different from that of the individual studies
cited. The overall goal of the process is the maximize CO2 capture while
minimizing energy consumption. Variations in the operating conditions of each
reaction should be investigated in order to quantify their effect on the entire
process. For example, heating the fluid in the causticization reactor representsa
compromise between increased energy consumption and longer reaction time.
Sophisticated designs may maximize the amount of heat recaptured at elevated
temperatures. For example, if the lime is hydrated using steam then the energy
output of this reaction increases by the heat of condensation. This would be offset
by steam generation elsewhere, but the higher quality heat obtained may be easier
to harness. In the end, the minimum theoretical penalty for calcination is 4.1 GJ
per tonne of CO2 and the maximum energy recoverable from hydration is 1.5 GJ
per tonne of CO2. The resulting minimum net energy penalty is 2.6 GJ per tonne
of CO2, which is already lower than the practical energy cost of amine solutions.~
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Theoretical efficiencies may not be achievable in practice but they are an
indication of the potential of the method.
REFERENCES
Abanades,J.C.,The maximumcaptureefficiencyof CO2using a carbonation!
calcinationcycle ofCaO/CaCO3,ChemicalEngineeringJournal, 90, 303-306
(2002).
Aroonwilas, A., P. Tontiwachwuthikul.,
A. Chakma, Effects of operating and
design parameters on CO2 absorption in columns with structured packings,
Separationand Purification Technology,
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